It could be a lot of things it could be his digestive track isn't working well, he is allergic to his food, you gave him something that upset his stomach or he is drinking too much water!! If it continues over a week take him to the vet diareahh isn't a good thing for a rodent PS there are drops you can get at pet stores that you put in the rodents water that prevent diarriah but consult a vet or a pet store clerk on it first